Electromagnetic-wave energyabsorbing material



BP 9, 1952 H. A. WHEELER 2,610,250

ELECTROMAGNETIC-WAVE ENERGY-ABSORBING MATERIAL Filed Nov. 5, 1946INVENTOR, HAROLD A. WHEELER,

ORNE

Patented Sept. 9, 1952 ELECTROMAGNETIC-WAVE ENERGY- ABSORBING MATERIALHarold A. Wheeler, Great Neck, N. Y., assignor to I Hazeltineltesearch,Inc., Chicago, 111., a corporation of Illinois Application November 5,1946, Serial No. 707,787

purely resistive wave impedance yet is capable of dissipatingsubstantial amounts ofwave energy. In such cases, it may be desirable toabsorb all or part of the wave energy. In the former category may i beclassified wave-energy terminating loads and in the latter wave-energyattenuators.

Dissipation of either the electric-field or magnetic-field energy of atranslated wave, however, causes a reactive component to appear in theotherwise purely resistive wave impedances of the wave-propagation path.The amount of the reactive component is proportional to the rate ofattenuation per Wave length of the translated wave.- 1

In all such wave energy-absorbing arrangements, it has heretoforeproven-exceedingly difficult to effect the desired wave-energydissipation without at the same time causing at least some, andfrequently a substantialamount, of waveenergy reflection by virtue ofthe presence of the energy-absorbing materialutilized. Such reflection:occurs if the wave impedance of the medium is notrpurely resistive buthas a reactive component.

Oneprior proposed arrangement for dissipating the energy of anelectromagnetic wave utilizes the so-called -lossy coaxial transmissionline. This line includes at least a conductor having substantialresistance or a dielectric material having substantial dissipationregarded as shunt conductance. To avoidreflection of Wave energy attheinput terminals of the line, the line is caused to have a substantiallypurely resistive input impedance by making its rate of attenuation per.wave-length so small that the resulting reactive-component of itswaveimpedance is negligible. .Thatis, such. lineshave heretofore beendesigned :for a small rate of attenuation so as to reduce the reactivecomponent below :a permissible tolerance. No attempt has been made tobalance thefltwo kinds of reactive components 5 Claims. (Cl. 17844)caused by the electric-field and magnetic-field energy dissipations. Itis therefore necessary to employ a great length of ossy line where it isessential that a substantial part or all of the wave energy appliedthereto is to be dissipated in the line. The resulting bulk and expenseof such a long line is a serious disadvantage in many applications.

When the frequency of a wave signal becomes so high that transmissionlines may no longer be conveniently used for wave propagation, a hollowconductor having parameters selected with relation to the wave length ofthe wave is conveniently used for this purpose. Energy-absorbingarrangements heretofore proposed for wave guides are in the, nature ofstrips of dielectric material or bodies of sand, either of which iscoated or impregnated with a resistive material such as collodialgraphite. The material is conveniently molded or otherwise shaped into aconfiguration so selected as to minimize reflection of wave energycaused by the presence of the material in the wave-propagation path.-This-result is accomplished by providing a rate of attenuation per wavelength which is tapered from a low value to a high value; It is verydifilcult to avoid such reflection in limited space, however,regardless-of the selected shape of the material; and this ,fact hasheretofore necessitated the use: of some additional means forcompensating or avoiding the efiect of the reflected Wave energy.Substantial values of energy dissipation in limited'space necessitatelarge values of energy attenuation per wave length of the translatedwave, so large in fact that dissipation of either'the electric-fieldenergy or the magnetic-field energy by itself would cause adetrimentalamount of reactive component'in the wave impedance presented to thetranslated wave. It may be mentioned that reflected wave energy, unlesscompensated as mentioned, is very undesirable in many applications forthe reason that it impairs the operation of wave-signal apparatuslocated along the wavepropagation path at points preceding the point ofwave-energy reflection. Additionally, it is apparent that wave-energyreflection impairs the efficiency of those arrangements which areintended to absorb all of the wave energy supplied thereto.

It is an object of the present invention, therefore, to provide a newand improved electromagnetic-wave" nergy-absorbing material which avoidsone or more of the disadvantages and limitations of prior suchmaterials. 7

2 It is a further object of the invention to provide a new and improvedelectromagnetic-wave energy-absorbing material which is capable ofdissipative wave-propagation path having a predetermined wave impedance,which is capable of absorbing substantial amounts of wave energy yet hasa substantially purely resistive war/aim pedance equal to that of thenon-dissipative wave- "propagation path." i In accordance with aparticular form of the invention, an electromagnetic-waveenergy-absorbing material, adapted to be interposed in awave-propagation path of which at leasta por- I tion has a predeterminedratio of dielectric c'on-' stant to magnetic permeability for apredetermined mode ofv wave propagation therethrough comprises aquantity of'dielectric material anda quantityof high-conductivitymaterial consist ing of particles having at least one-dimension which issmall in relation to the wave length of an electromagnetic wave to betranslated thereby. The relative quantities of the 'aforesaiddielectricand high-conductivity materials are soproportioned with relation totheir dielectric constants and magnetic permeabilities as to provide,for the above+mentioned predetermined mode of Wave ropagationztherethroirgna and :along the efere said-portion of saidfp'ath, a ratioof effective dielectric constant to effective magnetic permeability for;the energy-absorbing material 'substantially equal toytheabove-mentioned predetermined'ratio. "The energy-absorbing materialinchides a, substantial quantity of high-resistance materialdisp'ersedin .at least a portion ofthe .aforesaidzdielectrie material consisting"ofparticles having at least. one dimensionsma'll in m lation tov thewave-length of the translated vv'ave and..-efiective. to causesubstantial dissipation of li'heielfitlic energy of the translated'wave.

*For a betterunderstanding of the present 'inv'ention, together withother and further objects thereof, eferenc is had. to the followingdescription 'takenin-eonnection with the accompanying drawing; and.itsl'scopewillzibe pointed out in the appe ded c a ms 2* r "Referringnow'to the drawing, Fig; '1 illustrates an electromagnetic-waveenergy-absorbingmaterialie'mbodying the present invention and 11121-lized inia. particularapplication; Figs. 2 and-3 i1 lustratemodifiedforms. of :the invention; and Fig. 4 illustrates the use of: thematerial embodyingnthe. invention. in anotherapplication; Referringnoware more particularly tjoIFigL 1 there iisxillustratcd a,high-frequency electroma'g neti'crewave energy-absorbingmaterialembodying thepresentinvention inaparticular form. En' ergy absorbingmaterial to embodying the presentinventionis here shown by way ofexample as utilized in .a coaxialctransmission line which in-'- cludesanouter conductor and an inner con ductor l2, the latter being supportedby the material, 1H3 'incoaxialrelation with the conductor H11Thegtransmission line 11,- F2 is essentially a wave 'signa-l propagatingdevice: adapted to effect propagation of highefrequencya electromagneticposed in thewave-propagation pathfland com T material It.

in, the former conductive particles, for example combined iron or anycomminuted conductive material such as copper, brass, silver, 7 jaluminum, etc. Iron','of 'course, is recognized not --'-only'as aconductive material but also as a mag- 4 prises three materials; namely,dielectric material, conductive material and resistive material.

The conductive material is dispersed in at least a portion of thedielectric material and is of such configuration and conductivity as tocause dissipation of wave energy in substantial amount in a givenimannerwith reference to"thejdensity of the magnetic energy ofa'n'electr'omagnetic wave translated through the energy-absorbing Thisconductive material may be neticinaterial'inthat it has a coefficient ofmagneticpermeability greater than that of free space i Copper, silver,or the like are conductors not referred to as magnetic materials, butparticles thereof cause dissipation in accordance with the density of.magnetic energy inia waver; Usually the given manner. of energy,dissipation to which reference: was last made'is a proportional one.'I'h'atis; the; dissipation of the wave energy'bythe conductive materialis usually proportional to the density of the magneticenergy of thetranslated electromagnetic wave. v

- Theiparticles of the conductive material of the energy absorbingmaterial I!) are individually insulated and may be held together ,in acompact mass by a suitable dielectric binder.

The resistive material of, the energy-absorbing material lli'also isdispersed in atlea'st. a portion of. the dielectric materialpreviouslymentioned andvis of such configuration and resistivityastocause. dissipation of. I wave energy in substantial amount and in theaforementioned given manner, with'reference torthe'density ofthe'electric field of that wave translated through theenergy-absorbingniaterial l0. This resistive material: also may be: in.the form of 'individuallyinsulated: particles, fol-example=powderedcarbon or graphite, one preferred form being colloidalgraphite.v The relative concentrations of. the conductive and re sistivematerials areso proportioned as to -cause substantially. equal amountsof. the magneticfield-energy 'idissi-pationniand the, electric-field:-energy dissipation of the translated electromagnetic ..wave.- Thiscauses the wave impedance; of

the:energy absorbingmaterial If]. to be substantially purely resistive,because; the two, kinds of.

dissipation tend "to. cause opposite reactive com;- ponentslinthe waveimpedance.

The dielectric, ;conductive and resistivematerialsma'y bemixeditogetherin the-proper quane titles :into an isotropic ;-'mixtureand are then molded,fcast'orcompresseddnto1a. rigid mass ofconfiguration suitable for the use intended. 'lhus in a coaxialtransmissionline of-the type shown in: -Eig-.: l; the energyabsorbing;material: l 0 may;, after proper mixing of its ingredientmaterials,

be molded'or. compressedinto apertured cylindrii- I cal; blocks Y ordiscssuitable for. insertion between the linejconductors :l I Hi.vAlternatively, the material l0; may be extruded. upon the-line con,-ductor l;2 after which ;the ;line conductor 'HJmay be applied. overthematerial 1:0 as a woven braid or. tube. t When the energy-absorbingmaterial 1.0a

isrforme'd as described, theiparticle's of conductive and resistive:materials. are hleld in solid'zsusp'ene sionaf in, "and "dispersedthroughout, the: dielectric. materialso that the latter providesindividual-insulation-ior thep'articles. I a .The; particle si'zeandp'article spacingfor an isotropicmedium must be much less than :airadianlength. of the translated wave and, of course, the density must besubstantially uniform throughout the medium. Particles having a radiusof the order of one radian length in the material of the particle areperhaps somewhere near the optimum. In the case of metals and graphite,one radian length is equal to the depth of penetration in the material.Where the material is not isotropic, as a laminated form ofthe materialpresently to be described, the particles of conductiveand resistivematerials should nevertheless have somedimension much less than a wavelength of the translated wave signal. For magnetic material, thedimension along the vector of the electrical field of the wave should besmall; for resistive material, the. dimension along the vector of themagnetic field should be small. The term particle as used in the presentspecification and. claims is thus intended to mean pieces of materialhaving sizes small in relation to the wave length of the translatedwavesignal. Forvery long wave lengths, the magnetic material might thus.be scrap iron, sheets of con-. ductive material, and the like.

A Inthe operation of a transmission line of the type illustrated,electromagnetic wave energy whenapplied to one end of the line ispropagated between the line conductors and travels along the line. Themagnetic field of the electromagnetic wave sets up eddy currents in theconductive particles of the material and these currents dissipate someof the magnetic-field energy of the translated wave. The wave thusexperiences what may be considered equivalent to a series resistance toits propagation through the material I0. The particles of resistivematerial included in the energy-absorbing material I0 cause, on theother hand, dissipation of some of the. electric-field energy of thetranslated wave so that the resistivev material may be consideredequivalent to a shunt conductance between the line conductors ll, 12.

In this type of transmission line, it maybe desirable to use suchquantities of the dielectric, conductive and resistive materials as toprovide a desired ratio of effective inductance to eifective capacitancefor the line, in which case the invention contemplates a relativeconcentration of conductive and resistive particles to provide asubstantially equal ratio of equivalent series resistance to equivalentshunt conductance. When this is done, the transmission line has a waveimpedance which is substantially purely resistive. This result isobtained over a wide range of electromagnetic-wave frequencies if theparticles are sufiiciently small and closely packed. Thus if theparticles are so small that the radius of each particle is less than oneradian length in the material of the particle, the resulting attenuationcaused by the particle is proportional to the square of the frequency.If both kinds of attenuation are obtained by particles small enough tomeet this requirement, it follows thatboth kinds of attenuation areproportional to the square of the frequency and, therefore, the equalityof the electric field and magnetic field dissipations is maintained overa large frequency range. It fails only to the extent that other causesof attenuation become more important at lower frequencies, such as theresistance of the conductor in a transmission line. the power factor ofthe dielectric binder, etc. When'the material .ln has apurely resistivewave impedance, it is capable of dissipating a large part of theelectromagnetic wave energy in a small distance without causingreflection.

. It frequently is desirable that the energy-ab sorbing material of thepresent invention be interposed in a wave propagation path of which atleast a portion has a predetermined ratio of dielectric constant tomagnetic permeability which determines its wave impedance. Such a pathmay be non-dissipative and so have a purely resistive wave impedance.This need may occur, for example, whenf a transmission line of the typeshown in Fig. 1 is coupled to a transmission line which utilizes an airdielectric; between its conductors. An air-dielectric line has a unityratio of its'dielectric constant to magnetic permeability,'if both areexpressed relative to free space. If this ratio ofdielectric constant tomagnetic permeability is not maintained along the entire length ofthewave-propagation path, reflections of wave energy occur at those pointswhere there is experienced achange of the ratio mentioned. This isundesirable in many applications for wellknown reasons.

Fig. 2 illustrates a form of transmission line of the balanced typehaving spaced'conductors H, I2. The energy-absorption material I0 usedbetween the line conductors has such composition that most desiredratios of dielectric constant to magnetic permeability may be readilyobtained. The method here shown of forming the material 1.0 for, use ina balanced'line may also be utilized forracoaxial line of the Fig. 1type. The ma terial I0." here includes a layer of dielectric material :3having a lamination thickness a1, a dielectric constant 101, anda'coemcient of magnetic permeability n. It also includes a contiguouslayer of conductive material I 4 in the form of individually insulatedconductive particles held together' by a suitable'dielectric binder.This layer of conductive material 14 has a lamination thickness (12, aconstant lea-and a magnetic permeability ,u2. An electromagnetic wavehas its electric and magnetic fields normal to each other. A wave ofthis type is propagated through the material ID with such polarizationthat the magnetic field H is parallel to the layers of the materials I3,l4 and the electric field E is normal to the layers, as indicated by thearrows H and 'E in Fig. 2.

' "The laminated construction here utilized is disclosed and claimed inapplicants copendingapplications Serial No. 563,716, filed November 16,1944, now Patent No. 2,508,479, issued May 23, 1950, entitled-High-Frenquency Electromagnetic-Wave Translating Arrangement}? andSerial No. 563,715, filed November 16, 1944.,now Patent 2,511,610,issued June 13, 1950, entitled High-Frequency Electromagnetic-WaveTranslating" Element, both assigned to the same assignee as the presentapplication. As there explained, the thickness in of the layer ofdielectric material [3 and the thickness (12 of the layer of conductivematerial M are selected in relationto the dielectric constants I01 andkg and magnetic permeabilities 1 and ,u2 as to provide for the materialI 0' a'ratio of effective dielectric constant to effective magneticpermeability of the desired value. For reasons explained in applicantsaforementioned copending applications, the maximum lamination thicknessof the dielectric material l3 and of the conductive material I 4 shouldbe much less than one radian length of the translated electromagneticwave. As in the arrangement of Fig. 1, resistive material in the form ofparticles is dispersed in either or both of the dielectric mate'r'i'all3 and conductive material l4. Also as in Fig. 1, the quantity of theresistive material is such as to cause an amount ofelectric-fleld-energy dissipation equal .to the amount of magnetic-:field-energy dissipation caused by the .magnetic conductive materialduringthe propagation of the electromagnetic wave through the materialFig; 3 illustrates an energy-absorbing material 1.0" which isessentiallysimilar to that of Fig. 2,

similar elements .and materials being designated by :similar referencenumerals, except that the material 10" is formed of alternatinglaminations of dielectric material Island conductive material zl 4havingquantities relatively proportioned dielectric constant to effectivemagnetic permeability. As in Fig. :2, the maximum lamination thicknessof the dielectric and conductive materials should be much less than oneradia length of the translated wave signal.

.When the energy-absorbing material of the present invention isinterposed in a wave-propagation .path and provides unity ratio of theeffective dielectric constant to effective magnetic permeability asis'characteristic of free space, the wave-propagation velocity throughthe material is less than that of free space." Fig. 4 representsaneapplication of the present invention in which it'zis desired that thewave-propagation path through the. portion thereof occupied by theenergy-absorbingmaterialhave an effective electrical. length greaterthan its physical length by virtue of the reduced propagation velocitymentioned In this arrangement, the energy-absorbingtm'aterial IO is ofcylindrical cross section and is positioned on'thexinner conductor l2 ofthe transmission line I I, I2. Assuming theline to' be air .filled,uniformity 'of impedance along *the transmission line is maintained whenthe radial thickness of the material IO has a value given by therelation:

where rthe eficctive magnetic permeability of the Tmaterial of element1' kzthe effective dielectriccons'tant of the materialof element 40. v

The length of the material is selected, of course, to providethe desiredincrease of electrical length of the transmission line. As before, therelative concentrations of the conductive and resistivematerialsprovided in the energy-absorbing material IO' are'such as tocause substantially equal amounts of themagnetic-field-energydissipation and electro-field-energy dissipationsothat the wave impedance-0f the transmission line is substantiallypurelyresistive.

From the above description of the invention, it will be'apparent that anelebtromagnetic-wave energy-absorbing material embodying the present'invention is adapted to provide, in a limited space,substantial-absorption of electromagneticwavezenergy yet is onewhic'h'has a substantially purely resistive wave impedance and thus doesnot cause reflection of wave-signal energy. The

I magnitude of attenuation of which the material yet maintaining thewave impedance of: the ma-' asinr ig. 2 to 'attaina desired ratio ofeffective 7 B te'r ial substantially purely resistive. For example,it'is easily possible to attain with the material of the presentinvention an attenuation of about one napier per .radian'length ordecibels per wave length While-maintaining the net reactive component ofwave impedance of the material to within a few vper cent. of theresistive component;

with-only one kind of dissipatiomthat is dissipa- .tion of only theelectric-field energy or the magnetie-field-energy of the translatedwave signal,

While there have been described whatfare tat presentzconsidered to :bethe preferred, embodiments of this inventiomit will be jobviouszto thoseskilled in the art that various changesandmodifications may be madetherein without departing from the invention, and it is, therefore,aimed tin the appended claimsto cover all :such changes andmodifications as fall within the true spirit andiscope of the invention.l e What is claimed'is:

g 1. An electromagnetic-wave energy-absorbing material, adapted tobeinterposed in a "wavepropagation pathof which at least a portionhas apredeterminedratio of dielectric constant to magnetic permeability forapredetermined-mode of wavepropagation therethrough comprising: aquantity-ofdielectrical material; a quantity of high-conductivitymaterial consisting of particles having at least one dimension which issmall in relation to'the wave length ofanelectromagnetic wave tobetranslated thereby; the relative quantities of said'materials being soproportioned with relation to their dielectric constants, and magneticpermeabilities as to provide, for

material adapted'to be interposed in a wavepropagation path ofwhich. atleast a portion has a 1 predetermined ratio ;of dielectric constant tomagnetic permeability and a -substantially purely resistive waveimpedance for a predete'r mined'mode of wave propagation therethroughcomprising: a ciutntity of dielectric material} a substantial quantityof high-conductivity mate-- rial consisting of individually insulatedparticles having maximum dimensions which are small in relation-tethewave length of'an electromagnetic wave-to be translated therebyand-effective to cause substantial'dissipation or the magnetic energyorsaid translated wave the relative quan- -tities of said materials beingso proportioned with relation to their dielectric constants and magneticpermeabilities as to provide, for said predetermined mode of wavepropagation therethrough and along said portion of said path, a ratio ofefiective dielectric constant to effective magnetic permeability forsaid energy-absorbing material substantially equal to said predeterminedratio; and a substantial quantity of high-resistance material dispersedin at least a portion of said dielectric material consisting ofindividually insulated particles having maximum dimensions small inrelation to the wave length of said translated wave and effective tocause substantial dissipation of the electric energy of said translatedwave.

3. An electromagnetic-wave energy-absorbing material, adapted to beinterposed in a wavepropagation path of which at least a portion has apredetermined ratio of dielectric constant to magnetic permeability anda substantially purely resistive wave impedance for a predetermined modeof wave propagation therethrough comprising: a quantity of dielectricmaterial; a quantity of high-conductivity material consisting ofindividually insulated particles having maximum dimensions of the orderof one radian length of an electromagnetic wave to be translatedthereby; the relative quantities of said materials being so proportionedwith relation to their dielectric constants and magnetic permeabilitiesas to provide, for said predetermined mode of wave propagationtherethrough and along said portion of said path, a ratio of effectivedielectric constant to effective magnetic permeability for saidenergyabsorbing material substantially equal to said predeterminedratio; and a substantial quantity of high-resistance material dispersedin at least a portion of said dielectric material consisting ofindividually insulated particles having maximum dimensions of the orderof one radian length of said translated wave and efiective to causesubstantial dissipation of the electric energy of said translated wave.

4. An electromagnetic-wave energy-absorbin material adapted to beinterposed in a wave-propagation path of which at least a portion has apredetermined ratio of dielectric constant to magnetic permeability fora predetermined mode of wave propagation therethrough comprising: atleast one layer of a quantity of dielectric material; at least one layerof a quantity of highconductivity material contiguous to saidfirstmentioned layer and comprising individually insulated particleshaving maximum dimensions which are small in relation to the wave lengthof an electromagnetic wave to be translated thereby; the relativequantities of said materials being so proportioned with relation totheir dielectric constants and magnetic permeabilities as to provide,for said predetermined mode of wave propagation therethrough and alongsaid portion of said path,

a ratio of effective dielectric constant to efiective magneticpermeability for said energy-absorbing material substantially equal tosaid predetermined ratio; and a substantial quantity of highresistancematerial dispersed in at least a portion of said dielectric materialconsisting of individually insulated particles having maximum dimensionssmall in relation to the wave length of said translated wave andeffective to cause substantial dissipation of the electric energy ofsaid translated wave. 7

5. An electromagnetic-wave energy-absorbing material, adapted to beinterposed in a wavepropagation path of which at least a portion has apredetermined ratio of dielectric constant to magnetic permeability fora predetermined mode of wave propagation therethrough comprising: atleast one layer of a quantity of dielectric material having a maximumthickness much less than one radian length of an electromagnetic wave tobe translated thereby; at least one layer of a quantity ofhigh-conductivity material, contiguous to said first-mentioned layer,comprising individually insulated particles having maximum dimensionswhich are small in relation to the wave length of said translated waveand having a maximum thickness much less than one radian length of saidtranslated wave; the relative quantities of said materials being soproportioned with relation to their dielectric constants and magneticpermeabilities as to provide, for said predetermined mode of wavepropagation therethrough and along said portion of said Path, a ratio ofeffective dielectric constant to effective magnetic permeability forsaid energy-absorbing material substantially equal to said predeterminedratio; and a substantial quantity of high-resistance material dispersedin at least a portion of said dielectric material consisting ofindividually insulated particles having maximum dimensions small inrelation to the wave length of said translated wave and eifective tocause substantial dissipation of the electric energy of said translatedwave.

HAROLD A. WHEELER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

